We have recently seen increasing demands for memory devices with a large capacity to store multimedia information requiring increased bandwidth. Attempts are being made vigorously to increase the data density on, especially, rewritable optical and magnetic disks and magnetic tapes.
Japanese Laid-Open Patent Application No. 3-130904/1991 (Tokukaihei 3-130904; published on Jun. 4, 1991) discloses one of these technologies (conventional technology I) which is currently a focus of attention. In conventional technology I, data is stored on a magneto-optic storage medium in which a high temperature superconducting film and a perpendicularly magnetized recording film are stacked. To record data, a part of the superconducting film is heated beyond a critical temperature so that the diamagnetism disappears in that part of the superconducting film. This restricts the entry area through which magnetic fluxes extend into the perpendicularly magnetized recording film, and reduces the recording bit size accordingly. Thus, high density recording is effected.
Japanese Laid-Open Patent Application No. 4-176034/1992 (Tokukaihei 4-176034; published on Jun. 23, 1992) and Japanese International Publication for a Patent Application under PCT No. 6-500194/1994 (Tokuhyohei 6-500194; published on Jan. 6, 1994) disclose conventional technology II whereby data is magnetically recorded and reproduced with high density in narrow tracks by means of projection of assisting light onto the magnetic storage medium (hereinafter, will be referred to as light-assisted magnetic recording and reproduction technology). According to the technology, the storage medium is composed of a ferrimagnetic material that possesses a magnetic compensation temperature near room temperature. To record data, a laser beam is projected onto a track of the storage medium where data is to be recorded, raising the temperature of the recording layer in the track practically to the Curie temperature. Then, an external magnetic field is applied using a recording-use magnetic head, to effect the recording. To reproduce data, a laser beam is projected onto a track of the storage medium where data is to be reproduced, raising the temperature of, and thereby amplifying the magnetism in, the recording layer in the track. Then, the magnetic flux leaking from the recording layer is detected using a reproduction-use magnetic head, to effect the reproduction.
In the high density recording according to conventional technology I, the data stored in the perpendicularly magnetized recording film is reproduced by reading it either (1) by means of Kerr rotation effects caused by a laser projected onto the perpendicularly magnetized recording film from a source which is placed across the perpendicularly magnetized recording film from the superconducting film or (2) by means of magnetic flux leakage from the perpendicularly magnetized recording film detected using a reproduction-use magnetic head which is placed across the perpendicularly magnetized recording film from the superconducting film. Therefore, to reproduce high density data, either the laser beam or the magnetic pole of the reproduction-use magnetic head should be reduced in size to match the minuscule size of the recording bit.
Meanwhile, to read stored data using laser beams, a problem is encountered concerning refraction limits, where the lower limit of the beam size is dictated by the wavelengths of beams available for reproduction and the NA of an objective lens. Therefore, if a beam which is larger than the minuscule bit is used in reproduction, influence, i.e., crosstalk, from adjacent recording bits are difficult to keep under control. To reproduce data using a magnetic head, technical problems are encountered concerning microscopic fabrication of the magnetic pole in a magnetic head.
As described above, according to conventional technology I, it is difficult to reproduce signals with excellent S/N ratios while keeping influence, i.e., crosstalk, from adjacent recording bits.
According to conventional technology II based on the light-assisted magnetic recording and reproduction technology, the storage medium should be composed of an n-type ferrimagnetic material having a magnetic compensation temperature at room temperatures. Compared to those ferromagnetic materials used in ordinary magnetic recording without assisting light, the storage medium made of such a ferrimagnetic material exhibits only a fraction of residual magnetism in reproduction. Therefore, the storage medium cannot produce leaking magnetic fluxes that are sufficiently strong for detection by a magnetic head, resulting in insufficient signal strength and reduced S/N ratios in reproduction.
Besides, according to conventional technology II, residual magnetism, however small, does occur on a part of the surface of the storage medium where the above-mentioned requirement that the compensation temperature is at room temperatures is not fulfilled due to distribution of magnetic substances or any other reason. Leaking magnetic field produced by this residual magnetism is another cause of crosstalk, reducing S/N ratios in light-assisted magnetic reproduction as a result. As can be clearly understood from the description, a storage medium used according to the technology is required to have an extremely high degree of uniformity in composition. Also, in design, materials for the storage medium should be chosen from only a limited range of materials: no other materials but n-type ferrimagnetic materials that have a magnetic compensation temperature at room temperatures can satisfy conditions required in recording by the technology.
In view of the above problems, the present invention has an object to present magnetic storage media capable of performing high density data recording and reproduction and exhibiting excellent S/N ratios in reproduction, and also to present methods of recording and reproduction using the magnetic storage media.